1. Field of the Invention
The invention relates generally to precision moving tables, and load-supporting precision bearings; and more specifically to such devices that are stabilized magnetically.
2. Related Art
Both of my earlier patent documents identified above teach use, in mechanical drives, of couplings or bearings that typically transmit linear motion along a drive direction. These couplings may be said to xe2x80x9ccarryxe2x80x9d a load in the sense of load transmission, but not generally in the sense of support.
These couplings absorb lateral motions through rolling action of balls between coupling or bearing elements. At least one of these elements is a magnet that retains the balls between the elementsxe2x80x94and in some configurations helps keep the elements in line. The magnet also creates compressive constraint along the drive direction; this constraint prevents backlash.
The first of my two earlier patent documents identified above relates particularly to drives in which linear motion along the drive direction is derived from rotary motion about an axis parallel to that same drive direction. The second document relates to specific coupling configurations that typically transmit torque as well as longitudinal linear motion.
The present document is not directed either to the context of rotary drives or to the transmission of drive force or torque. It does, however, employ some devices that are related to wobble-absorbing bearings and couplings disclosed in those earlier patent documents.
For brevity and simplicity in this document some terminology is used in a manner that may be partially specialized:
In accordance with standard practice in discussing practical systems, the terms xe2x80x9ccylindricalxe2x80x9d, xe2x80x9ccylinderxe2x80x9d, xe2x80x9csphericalxe2x80x9d and xe2x80x9cspherexe2x80x9dxe2x80x94except where context otherwise indicatesxe2x80x94refer to surfaces and articles that either are formed as portions of cylinders and spheres, not necessarily entire cylinders and spheres, or that relate to cylinders and spheres.
Thus for instance a xe2x80x9ccylindrical axisxe2x80x9d is the axis of a cylinder or part of a cylinder, a xe2x80x9cspherical centerxe2x80x9d is the center of a sphere or part of a sphere, etc.
To a certain extent the terms xe2x80x9ctablesxe2x80x9d and xe2x80x9cload-supporting bearingsxe2x80x9d are interchangeable. A small table may be semantically and functionally indistinguishable from a large bearing.
Generally the invention taught in this document deals with load-supporting bearings, as distinguished from other sortsxe2x80x94such as drive bearings, rotary-motion axle supporting bearings, etc. Therefore, except where context suggests otherwise, references to xe2x80x9cbearingsxe2x80x9d of this invention, in the text and appended claims of this document, encompass like structures used as tables; and conversely.
The word xe2x80x9cbearingsxe2x80x9d encompasses laterally guiding devices that may sometimes operate entirely or partly in tension, as well as compressive-support bearings per se.
In this regard a support that takes the form of a suspension device may be traditionally regarded as a xe2x80x9cbearingxe2x80x9d; for present purposes such a suspension device may as well be a xe2x80x9ctablexe2x80x9dxe2x80x94even if articles positioned along the table are actually hanging from it.
The word xe2x80x9cwobblexe2x80x9d is used in an extremely general way, to encompass any spurious lateral motionxe2x80x94such as vibration, play, and jitter, that are lateral with respect to an intended direction of force transmissionxe2x80x94as well as spurious motions that are generated incidentally to a desired rotation and therefore perhaps more classically identifiable as wobble.
Accordingly, the lateral-motion-absorbing devices of my earlier patent documents as well as this one may be conveniently called xe2x80x9cwobble-absorbing magnetic bearingsxe2x80x9d, or xe2x80x9cWAMxe2x80x9d bearingsxe2x80x94or simply xe2x80x9cWAMBsxe2x80x9d. In this document, reference to such WAM bearings encompasses the varieties disclosed in those earlier documents as well as those disclosed here.
My earlier patent documents discuss an invention of Norris, a rotary-motion bearing with ferromagnetic balls that are held in place without a bearing spacer or bearing retaining-ring holder by making one of the bearing surfaces magnetic. Norris does not teach a wobble-absorbing bearing, or a load-carrying (in the sense of load-supporting) bearing or table.
In addition to the art cited in, and in connection with prosecution of, my above-identified earlier patent documents, I have noted the following materials which may be of interest:
U.S. Pat. No. 3,720,849 Bardocz
U.S. Pat. No. 5,407,519 Joffe et al.
U.S. Pat. No. 5,380,095 Pryor
U.S. Pat. No. 5,237,238 Berghaus
U.S. Pat. No. 5,001,351 Boksem.
Bardocz deals with improving the positioning precision of a ball-mounted moving table through magnetic constraints. He mentions that backlash along a drive direction too can be removed through magnetic constraint.
Bardocz is not at all specific about the manner in which his tables are magnetically constrained; thus his teachings in some regards are rather incomplete. There is some evidence that Bardocz""s teachings may have been elaborated and refined commercially, as shown by FIG. 49, which is copied from a scientific-instrument journal (unidentified) circa 1987.
The drawing shows that balls roll along hardened steel inserts in V-grooves, and in one position a flat groove, formed in opposing surfaces of a table xe2x80x9cpositionerxe2x80x9d mechanism. Magnets are shown inserted laterally inboard of the grooves, in the lower surface, to attract ferromagnetic inserts in the upper surface.
An accompanying graph (not reproduced here) shows xe2x80x9cTypical tracking precisionxe2x80x9d of the devicexe2x80x94height variation along a 12 mm path. The graphed values range generally within very roughly xc2x10.2 micron.
Accompanying text explains that xe2x80x9cthe whole positioner is held together by magnetic forcesxe2x80x9d, and describes the function of the magnetic inserts:
xe2x80x9cThese pieces, separated by only a few tenths of a millimeter, pull strongly toward each other. As the magnetic forces do not change much for small variations of distance, the force pressing the positioner together remains constant even considering the . . . disadvantages of rolling elements. A symmetrical movement is guaranteed as the strain on the whole positioning range remains constant . . . .xe2x80x9d
Study of the drawing makes plain that the strongest magnetic forces, being aligned with the magnets, are offset inboard from the load-bearing balls.
The opposing horizontal surfaces, however, are not directly supported (i.e., held apart) in the region between the balls. As a result the magnetic attraction tends to deform the illustrated structure, bowing the opposing horizontal surfaces together in the unsupported region.
Such distortions can create significant variations in elevation (and to a lesser extent angle) along the upper surface of the table. If apparatus is mounted over a significant span of that upper surface, the table distortions can mechanically induce corresponding distortions and stresses in that apparatusxe2x80x94potentially leading to spurious responses, not readily recognized or traced, of the apparatus.
Also, this article excerpt contains no suggestion that the technology might be pertinent to nonplanar surfaces. At this writing I have been able to learn no more about commercialization of the Bardocz invention; to the best of my knowledge no such effort has incorporated the claimed invention of the present document.
The Pryor patent may be truly termed the Pryor art, but by virtue of the earlier filing date of my ""743 application the Pryor art is not prior art with respect to that part of the subject matter herein which is disclosed in my U.S. Pat. No. 5,331,861. Pryor too relates to magnetic constraint of moving tables, and analogous modules such as drawer slides; as he says at the outset, however, he is not concerned with extremely high precision.
Pryor uses individual balls that either slip in setscrew ball nests and roll on opposing surfaces, or bind in the nests and slip on the opposing surfaces, or slip both on the nests and on the opposing surfaces. None of Pryor""s ball elements is fully rollingxe2x80x94i.e., able to roll at both sides of its interface.
Wobble (as above defined) between two interfacing elements (e.g., stages) of tables and bearings can have both translational and rotational components. It is a problem in many types of mechanisms because it causes small but significant errors in the work process being performed.
A table element (stage) or like object can be moved into a stationary position on a nominally planar guide surface, suspended on rollable balls kinematically as Bardocz teaches and the above-discussed diagram shows (or by setscrew-positioned sliding balls as Pryor teaches). In this case the object or stage may be subject to rather excessively high loading of each ball.
If instead multiple supports are provided, each extending over a wide range of positions, inevitably mismatches occur between nominal and actual shapes and orientations of the surfaces of the several supportsxe2x80x94and of the nominally flat guide surface too. Such mismatches introduce positioning error or uncertainty and in some cases even ambiguity, i.e. multivalued positioning functions, hysteresis etc.
All such disturbances are collectively termed xe2x80x9cwobblexe2x80x9d for present purposes. As suggested above they typically include spurious small rotational inconsistencies in orientations, as well as inconsistencies in average elevations, of arbitrarily selected support sites.
Like sources of error are often present in undersurfaces of (or within mechanisms associated with) the driven table or object. Therefore, in particular, rotary mismatches between support sites on the guide surface and sites under or in the driven object may be compoundxe2x80x94i.e., characterized by rotations about more than one center.
All these limitations are exacerbated where tables are not only positioned on but moving over the guide surface. All such drawbacks represent a problem in the art of precision tables and bearings.
My invention solves this problem easily, simply and inexpensively. In most instances it does so with only a minor increase in complexity of the apparatus.
According to the invention, no effort is made to remove either wobble effects or irregularities that are present in a guide surface or the undersurface of a mating driven object. Nevertheless adverse effects upon the workpiece or other movable object, and upon other elements of the system, are substantially eliminated.
Some of these benefits are obtained through certain details in the application of magnetic force to stabilize a table or bearing. Other benefits rely on use of one or more WAM bearings as supportsxe2x80x94either static or dynamic.